The present disclosure relates to a coated high-temperature superconducting wire using a high-temperature superconducting wire and to a high-temperature superconducting coil including the coated high-temperature superconducting wire.
As in Japanese Patent No. 4187293, Japanese Patent No. 4752744, and Japanese Patent Publication No. 2011-108918, a high-temperature superconducting coil using an oxide superconductor has been conventionally known. Among high-temperature superconducting wires, a rare-earth-based (ReBCO) wire called a “second-generation wire” has the following characteristics: a smaller thickness than that of a first-generation high-temperature superconducting wire (bismuth-based wire); excellent strength in a longitudinal direction; a great high-magnetic-field critical current density in engineering; and excellent electrical conductivity. For, e.g., a nuclear magnetic resonance (NMR) magnet used in an extremely-high magnetic field, the high-temperature superconducting wire is more advantageous as compared to a conventional low-temperature superconducting wire. A value for hoop stress generated on the high-temperature superconducting coil by electromagnetic force is obtained by the following expression: Magnetic Field Value×Current Density×Radius. Since mechanical strength in the longitudinal direction (lengthwise direction) is high, a current density can be high. This allows significant reduction in size of the high-temperature superconducting coil. As a result, e.g., the following can be realized: easing of installation conditions; reduction in amount of helium to be consumed; and reduction in material cost.
The high-temperature superconducting wire is extremely susceptible to bending. If the high-temperature superconducting wire is curved, bent, or twisted so as to have a bending diameter equal to or less than the minimum allowable bending diameter at which superconducting performance is not degraded, the superconducting performance is irreversibly degraded. For such a reason, in order not to cause the bending in a fabricating process, careful processing is required. In addition, the high-temperature superconducting wire is more susceptible to stress in a transverse direction as compared to a typical copper wire, and therefore the superconducting performance is irreversibly degraded due to the stress in the transverse direction. Further, the performance of the high-temperature superconducting wire is degraded due to exposure to a high temperature of equal to or higher than about 250° C. for a long period of time. A series of the following steps is performed one time or several times as a method for electrically insulating the high-temperature superconducting wire: a coating material (e.g., formal resin coating, polyester resin coating, polyamide-imide resin coating, and polyimide resin coating) for an enamel wire is applied to an outer surface of the wire, or the wire is dipped in the coating material; and the coating material is baked on the wire. Alternatively, an insulating tape such as a polyimide tape may be wound around the outer periphery of the superconducting wire.